The L2C leukemia is a B cell leukemia of inbred strain 2 guinea pigs. These cells have surface IgM and C3 receptors. Studies have shown that these leukemia cells possess a strong tumor specific transplantation antigen (TSTA) that can easily be demonstrated by immunization protection tests in syngeneic animals. A procedure employing KC1 extraction of the leukemic cell yields a soluble extract that is also highly antigenic. The physical and chemical properties of this soluble TSTA are now the subject of study. The findings to date indicate that this TSTA has several unusual properties; it has a M.W. of 12,500, (as determined by Sephadex chromatography and polyacrylamide gel electrophoresis); it is resistant to boiling for 5 minutees as well as to extremes of pH. Iso-electric focusing indicates that the immunogenic material has a high isoelectric point, between 9-10. Treatment with trypsin, neuraminidase and periodate destroys the activity. The fraction containing the TSTA was pink suggesting that it might be cytochrome c. However, immunization with mouse cytochrome c (having the same sequence as guinea pig cytochrome c) failed to protect. Furthermore, fractionation of cells demonstrated that fractions containing membranes protected while the cytosol soluble fraction failed to protect. These results suggest that the TSTA is not cytochrome c but that it is another small, basic glycoprotein.